This invention relates to molds for injection molding battery containers, the walls of which have thin, resilient, integrally molded ribs projecting at acute angles into the cell compartment(s) defined by the walls. The ribs deflect various amounts according to the thickness of the cell elements comprising the battery's electrochemical innards and serve to firmly retain the innards in the container while cushioning them against vibration. While the invention is most particularly useful for molding multicell, lead-acid, automotive battery containers, it is also applicable to single cell and other types of batteries.
Automobile, SLI, lead-acid storage batteries have standardized outside dimensions in different size categories. The manufacturers thereof typically provide batteries with varying energy capacities in each size category. This is done by varying the number of positive and negative plates in each cell element which comprises the battery's electrochemical innards. Varying the number of plates, however, varies the thickness of the cell element to be positioned and firmly retained in the container's cell compartment(s). Lower energy batteries typically contain about nine plates per cell element while higher energy batteries can contain as high as sixteen plates per element. These elements typically vary in thickness from about 2.42 cm for the low energy batteries to about 4.24 cm for the high energy batteries, depending on the number of plates and the particular manufacturer's specifications. In most commercial batteries today, each cell element is spaced from the walls defining the cell compartment by relatively thick (e.g., ca 1.5 mm) ribs integrally molded normal to the walls which ribs engage the cell elements and hold them firmly in the center of the compartment. Larger ribs (i.e., in greater relief from the wall) are used for the thin cell elements and conversely smaller ribs for the thicker cell elements. These ribs are simply formed by cutting slots into the solid cores of the mold.
To provide a variety of battery models, many battery manufacturers inventory at least one case mold for each battery model and have to shut down their production lines to change over from one model to the next. Other manufacturers inventory a lesser number of molds, but provide additional inert spacers (e.g., extra separators) in each cell element to add thickness to the cell elements having fewer plates. This approach, however, adds the extra cost of the spacer and frequently complicates the in-plant handling of the cell element particularly during insertion into the container.
At least one manufacturer molds oversized ribs in a few standardized containers and then cuts the ribs back to the desired size depending on the thickness of the cell element destined for the particular container. This technique is disclosed in U.S. Pat. Thune No. 4,041,603, issued Aug. 16, 1977 and assigned to the assignee of the present invention. This approach requires a separate trimming operation and tools therefor.
Still other manufacturers have proposed molding thin (ca. 0.6 mm) resilient ribs at angles to the container walls which ribs deflect by an amount commensurate with the thickness of the cell element inserted in the cell compartment. However, molding containers of this type is difficult if the ribs are to have any substantial size (i.e., height and extension). In this regard, it is difficult, at best, to accurately machine thin, angled, slots into a mold core with conventional tools without creating some undercuts and backdrafts; and to polish the surfaces of such slots sufficiently to reduce drag on the ribs during container ejection. Even when relatively smooth-walled slots are provided, the large area of rib-forming mold surface relative to the thickness of the ribs creates sufficient resistance to stripping that the force required to free the ribs from the narrow slots often stretches and deforms the ribs. Occasionally, the ribs tear leaving portions thereof lodged in the slot which then necessitates shutdown and cleaning of the mold. Finally, experience has shown that such molds require excessively long molding cycle times: to insure adequate filling of the thin, poorly-vented, rib-forming cavity at the beginning of the cycle; and to free the ribs from the cavity without distortion during container stripping at the end of the cycle.
Accordingly, it is an object of the present invention to provide a commercially practical mold for injection molding a thermoplastic battery container having thin, inclined, resilient, retainer ribs projecting into the container's cell compartment(s), which mold can be readily machined and polished as well as operated at commercially practical molding cycle times and all without stretching, tearing or otherwise substantially deforming the ribs while stripping the container from the mold. This and other objects and advantages of the present invention will become more readily apparent from the detailed description thereof which follows.